In one recently proposed lithium ion capacitor, lithium ions are occluded in and supported by a negative electrode capable of occluding and deoccluding lithium ions in advance using a chemical or electrochemical method to reduce the potential of the negative electrode, and high energy density is thereby achieved (see Patent Literature 1).
In this lithium ion capacitor, electrode layers containing active materials and formed on the front and back surfaces of current collectors are used as the positive and negative electrodes.
The conventional lithium ion capacitor, however, has a problem in that a reduction in capacity retention ratio and an increase in internal resistance occur after high-load charging-discharging is repeated many times. Another problem is that lithium precipitates on the negative electrode, causing a short circuit. Since the conventional lithium ion capacitor has such problems, when high-load charging-discharging is repeated many times, it is difficult to achieve long service life.